All the basic configuration of prior hobs (see FIG. 1) is similar to a screw-shaped worm. On the stem of the worm, flutes are made to form the rake face E, and still further relieving is performed to form the top flank F and the side flank F' and F" which are located at both sides. These side flanks and rake faces intersect to form the cutting edges. Although this kind of hob is very versatile, highly effective, and used widely, most of prior hobs need relieving to produce relief angles and the worm of the hob often adopts the other kinds of worms to replace the involute worm which is required by the theory, which makes the hob itself exist configuration error, and the machined gear error. Moreover, because on the prior hob the flutes are provided by cutting off the helix of the worm for shaping rows of discontinuous teeth, during hobbing there will be a shock. If the gear hob is made of carbide, it is prone to break since the carbide is brittle. And also because the cutting teeth are not continuous, in fact the machined profiles consist of broken lines, this damages the shape of the tooth profile. What is more, on the prior hob the relief angles of the side flanks F' and F" are very small namely only about 3.degree., less than a reasonable relief angle of 6.degree.-15.degree., therefore tool life is pretty low. As for a finishing gear hob, because chips are thin, hob wear appears on the flank mainly, and a relieved hob whose flank was relieved can only be resharpened on the rake face E. The resharpened surface is not reasonable. This is bound to make the allowable resharpening times decrease, and after resharpened tool precision varies. This restricts the allowable number of resharpening and makes the available total life of the hob shorter. It is pretty complex for the prior precise hob both in manufacturing and measuring. And the measuring items are more and strict, and the cost of measurement is rather high.
Therefore, it is the purpose of this invention to present a new precise hob for finishing the cylindrical involute gear to overcome the mentioned shortcomings which are in the prior finishing hobs, on which is provided the continuous spiral cutting edges that can meet the requirement of finishing the cylindrical involute gear, and are especially appropriate for finishing the profile face with high hardness.